Frequency-domain equalization is performed in some communications systems, such as communications systems based on orthogonal frequency domain multiplexing (OFDM) and single-carrier systems with frequency-domain equalization (SC-FDE). An OFDM system transmits multiple data symbols at the same time over a media on a plurality of sub-carriers in a frequency band. The sub-carriers (both pilot and data sub-carriers) with the same time index together make up an OFDM symbol.
When a transmitted OFDM signal is received by a receiver, the received signal is somewhat different from the transmitted signal due to the channel (typically a frequency-selective channel) and the noise. Discounting noise, the transmitted data symbols can be known from the received signal if the frequency response of the channel (also called “channel frequency response”) is also known. OFDM receivers typically estimate channel frequency response (also called “channel estimation”) in order to obtain the transmitted data symbols before demodulation is performed. Since the OFDM channel is typically frequency-selective and time varying, channel estimation is performed continuously and the received OFDM sub-carriers are equalized using the estimated channel frequency response.
FIG. 1 is a block diagram depiction of a conventional wireless OFDM receiver 100 which employs frequency-domain equalization. Receiver 100 receives an RF signal in the form of symbols including a plurality of sub-carriers (tones) via antenna 101 as a sample data stream from the left to the right of the FIG. The analog front end (AFE) block 105 receives the signal from the antenna 101, which may include a variable gain amplifier (VGA), down-converter (or mixer), RF/analog filters and analog-to-digital (A/D) converter. Cyclic prefix (CP) removal block 108 after the AFE block 105 can be used to remove the doubled (redundant) CP on an OFDM frame preamble structure for the IEEE 802.15.4 g standard having the LTF structure 300 including two LTF symbols shown in FIG. 3 as described below. More generally, in OFDM systems (including wireless local area networks (LANs)), CP removal block 108 removes the single CP included in the preamble.
After CP removal by CP removal block 108, the samples are then converted from serial to parallel (1 to N) by serial to parallel (S/P) conversion block 110. The receiver 100 includes a FFT block 115 which performs a Fast Fourier Transform (FFT) on the data from S/P conversion block to generate a plurality of frequency-domain samples, and then feeds the results to the frequency-domain equalization block 120. Frequency-domain equalization block 120 receives channel estimates for the respective sub-carriers from the channel estimation block 125 shown.
Frequency-domain equalization block 120 is operable to compensate the received signal for linear distortion from the channel (e.g., multipath effects). Error detection, decoding and parallel to serial conversion is performed by error decoding parallel to serial (P/S) conversion block 130, followed by symbol demapping block 135, then forward error correction (FEC) decoding block 140 to recover the original (transmitted) data stream.
In real-world systems, the signal processing modules (all blocks except antenna 101 and AFE block 105) are controlled by software run by a computing device, such as a digital signal processor (DSP) or application specific integrated circuit (ASIC). Although some channel estimation algorithms are available which generally provide good performance (i.e. accurate channel estimates), such as “optimal” minimum mean square error (MMSE)-based estimation algorithms, optimal MMSE requires different filter coefficient vectors each comprising a plurality of filter coefficients for processing each of the sub-carriers, particularly for sub-carriers toward the edge tones, and therefore involves high complexity algorithms and a high memory requirement, which generally become high cost solutions.